Generally speaking, an electric machine comprises a stationary part, commonly referred to as “stator section” (or simply “stator”), and a mobile part, both equipped with windings of electrical conductor and/or sources of a magnetic and/or electromagnetic field. Together with the machine structure, these windings and sources always form both an electric circuit (defined as an assembly of structures and materials with an electric current and/or an electric field flowing through it) and a magnetic circuit (defined as an assembly of structures and materials with a magnetic field flowing through it). In order to operate, the electric machine uses electromagnetic induction (produced by the concatenation of magnetic field fluxes with the electric windings) and/or electromagnetic forces (generated by the magnetic/electromagnetic field sources on the electric windings with current flowing through them and/or on the other magnetic/electromagnetic field sources). Some electric machines (for example, electric motors) can convert the electric current circulating in the electric windings into movement of the mobile part relative to the stator section. Other electric machines (for example, generators) can generate electric current and/or electromagnetic force in the electric windings using the motion of the mobile part relative to the stator section. An electric machine of this kind can normally be used in both ways (that is, as a generator and as a motor). The windings can be made around a core of magnetic material in order to optimize the effect of magnetic flux concatenation with the electric windings themselves.
In one type of electric machine, the mobile part is a rotating member, also known as “rotor section” (or simply “rotor”). The axis of rotation of the rotor section is particularly important and is usually used as the reference and/or symmetry axis for the structure of the electric machine. As the rotor section moves relative to the stator section, portions of the magnetic field sources and portions of the electric windings face each other at a certain distance defining a gap between the rotor section and the stator section. There is a geometrical relation between the axis of rotation of the rotor section and the pattern of the flux lines of the magnetic field, generated by the sources, in the gap between the stator section and the rotor section. Based on this geometrical relation, machines of this kind can be broadly divided into two categories: radial flux electric machines and axial flux electric machines.
Of these two types of electric machines, radial flux electric machines are the most widespread and well known.
In this specification, the term radial flux electric machine is used to mean: an electric machine where the arrangement of the magnetic field sources and of the electric windings with which the magnetic field is concatenated, is such that in the gap between the stator section and the rotor section (where source portions face winding portions during the motion of the rotor section) the magnetic field flux lines can be likened to line segments perpendicular to the axis of rotation of the rotor section and arranged radially with respect to the axis of rotation itself.
The term axial flux electric machine, on the other hand, is used to mean: an electric machine where the arrangement of the magnetic field sources and of the electric windings with which the magnetic field is concatenated, is such that in the gap between the stator section and the rotor section (where source portions face winding portions during the motion of the rotor section) the magnetic field flux lines can be likened to line segments parallel to the axis of rotation.
This invention addresses electric machines of the axial flux type.
The most common type of axial flux electric machine comprises: a generally toroidally-shaped stator section and at least one disc-shaped rotor section facing one of the two bases of the toroid constituting the stator section. In some cases, the machine has two rotor sections, each facing one of the bases of the stator section. Some electric machines may comprise two or more stator sections, alternated with respective discoidal rotor sections. Stator section and rotor section are coaxial along the axis of rotation of the rotor section. The rotational shaft of the rotor section generally passes through the central hole in the toroid constituting the stator section.
Usually, the rotor section mounts the magnetic field sources, preferably in the form of permanent magnets, while the stator section mounts the electric windings with which the magnetic field is concatenated. The magnetic field sources are normally distributed in a circular crown of the rotor disc that faces one of the bases of the toroid constituting the stator section.
The stator section of an axial flux electric machine usually comprises a toroidally-shaped core having a cylindrical outside lateral surface and a cylindrical inside lateral surface, coaxial with each other along an axis that coincides with the axis of rotation of the rotor section. Along this axis, the core is delimited by a first and a second base. The core is made of a magnetic (preferably ferromagnetic) material. The electric windings are in the form of a plurality of coils, equally spaced from each other along the annular centre line of the core, and electrically connected to each other in various ways. Each coil has a through hole around which the conductor that forms it is wound. The solid part of the core goes through the coil by way of the through hole in the coil itself. The coil thus has a first side extending along the outside cylindrical surface, a second side extending along the inside cylindrical surface, a third side transversal to the first two sides and joining them across one of the two bases of the core, and a fourth side transversal to the first two sides and joining them across the other of the two bases of the core. The distance between the two cylindrical surfaces of the core is greater than the distance between the two bases, and the first and second sides are shorter than the third and fourth sides. Since the machine is of the axial flux type, the third and/or the fourth side of each coil (extending from the outside cylindrical surface to the inside cylindrical surface on the base surfaces of the core) are the portions of the winding that face the magnetic field sources (for example, the magnets) during rotation of the rotor section. Thus, they cross the magnetic field in a region where the flux lines of the magnetic field are parallel to the axis of rotation of the rotor section (this axis coinciding with the shared axis of the two cylindrical surfaces of the core) and they are substantially perpendicular to the flux lines in that region. For this reason, the third and fourth sides are called the “active sides” of the respective coil (that is to say, the sides which, in the case of a motor, when current flows through them, are subject to magnetic forces that are able to induce rotation of the rotor section). The first and the second sides, on the other hand, are referred to as the “heads” of the coils. Usually, the coils have a radial orientation, that is to say the four sides extend along a closed curve that lies in a plane which in turn contains the axis of the two lateral cylindrical surfaces of the core. In particular, the two active sides—i.e. the third and the fourth side—extend in a radial direction, while the heads—i.e. the first and the second side—are parallel to the axis.
On account of the thickness of the electrical conductor winding, each of the four sides of a coil extends away from the core for a certain distance along the perpendicular to the surface of the core on which the side itself is located.
The base surfaces of the core may be flat (each one lying in a plane perpendicular to the shared axis of the inside and outside lateral cylindrical surfaces of the core). In this case, between the two third sides of two consecutive coils and between the two fourth sides of two consecutive coils there are empty spaces (air gaps). In this case, the core of the stator section of an axial flux machine is said to be “slotless”. Alternatively, the space between the third or the fourth side of one coil and, respectively, the third or the fourth side of the consecutive coil along the annular centre line of the core may be filled by a protuberance (also known as “tooth”), which protrudes from the core along the shared axis of the lateral cylindrical surfaces for a certain distance and extends in length from the outside lateral surface to the inside lateral surface. The third and/or the fourth side of each coil is thus positioned inside a groove (or “slot”) between two consecutive teeth. In this case, the core of the electric machine is said to be “slotted”. The teeth are also made of a magnetic material and make it possible to minimize the magnetic reluctance of the gap between one coil and the next in the active region of the machine (that is, in the region of the “active sides” of the coils), thus maximizing the efficiency of the electric machine. Usually, the teeth are made as a single part with the core.
Whether the cores are slotted or slotless, there is always an empty space (air gap) between the heads (first and second sides) of two consecutive coils.
To reduce eddy currents in the core (these currents, which reduce the efficiency of the electric machine, tend to be generated in the core along rings that surround the magnetic field lines), the core is usually made by winding a metal strip spirally on itself around the shared axis of the lateral cylindrical surfaces of the core itself. In this way, the interfaces between one strip and the next are distributed crossways relative to the path that would be followed by the eddy current rings, thus tending to break it and to reduce its effect. To make the teeth as a single part with the core, the strip, before being wound on itself or during the winding operation, is punched in such a way that slots and teeth are automatically formed when it is wound on itself.
The electric machine also comprises a casing (or enclosure) which the stator section is usually fixed to and which surrounds at least the latter around its axis.
During operation of the electric machine, power losses occur in the electrical circuit and in the magnetic circuit and, more specifically:                “copper losses” (that is to say, power losses in the electrical circuit of the machine, due to the Joule effect created mainly by the current flowing through the windings and in the electrical conductors);        “core losses” (that is to say, power losses in the magnetic circuit of the machine due mainly to the magnetic hysteresis of the magnetic materials and to eddy currents generated in the active parts of the machine, especially in the stator section, that is, core and coils).        
These power losses result in heat being given off. This heat must be removed to the surrounding atmosphere as effectively as possible because excessively high temperatures in the active parts of the machine (core and coils in the stator section) can damage and reduce the efficiency of the electrically insulating components, which are the most sensitive in terms of temperature limits.
Usually, the heat is removed from the contact surface between the stator section and the casing that houses the active parts of the machine and is dispersed into the atmosphere by the casing itself.
United States patent document U.S. Pat. No. 7,332,837 B2 discloses a stator for an axial electric machine with an attached cooling system. More specifically, the stator section comprises a toroidal core. Coils of electrical conductor are interspersed along the annular centre line of the core, each coil being spaced from the others and lying in a radial plane containing the axis of the toroid. The stator section comprises an outer annular casing of metal (preferably aluminium), which surrounds the core from the outside and remains coaxial with the core itself. The casing has teeth extending radially inwards into the gap between the outer head of one coil and the outer head of the next coil along the annular centre line of the core (that is, between the first side of one coil and the first side of the next coil, the first side of a coil being, as mentioned above, the one that extends on the outer cylindrical surface of the core). The teeth on the casing act as cooling fins. The spaces between the teeth and the coils are preferably filled with a filling material having good thermal conductivity. The body of the annular casing has an interior cooling channel, also annular in shape, that circumferentially engages the outside of both the core and the coils. A liquid coolant flows inside the cooling channel.
The stator section for a liquid-cooled axial flux electric machine as described above is not free of disadvantages.
In particular, the side faces of the outer heads of the coil are cooled only by contact with the metal teeth of the casing, while only the outer end of the head is affected more closely by the passage of the liquid coolant. In addition, the casing has a complex structure that is difficult to make. Machining the side of the casing that faces the outer heads of the coils is a particularly complex process. The interior annular channel is also difficult to make and must be adapted for each different stator size and type.